1. Field of the Invention
This invention relates to a security system using sensors.
2. Background Art
A well-known security system using sensors consists of sensors installed at a number of security zones, surveillance terminals installed at the respective security zones for forwarding security data based on the operation of the sensors, and a surveillance center for monitoring the individual security zones utilizing the security data received from the surveillance terminals.
Each of the conventional surveillance terminals used in this type of security system is configured to send prescribed security data to the security surveillance center via a telephone line or the like in response to the outputs of window sensors, door sensors, glass shatter sensors, infrared (body heat) sensors and/or various other sensing devices for detecting the intrusion of unauthorized persons into a prescribed security zone demarcated in, for example, a shop, store, financial institution, office or other such location. The surveillance center uses the security data to monitor intrusion of unauthorized persons into security zones.
When a surveillance terminal incorporated in the conventional security system using sensors of the foregoing structure breaks down or experiences a malfunction, security data that the surveillance terminal installed at a monitored facility should immediately send to the surveillance equipment at the surveillance center in response to sensor signals etc. is liable to arrive at the surveillance center late, so that the surveillance center does not receive the required security data at the required time, or not to be transmitted at all.
Since the conventional security system using sensors does not check whether received security and other data arrived after a delay, an intrusion detected by the sensors at a monitored facility may not be ascertained by the security surveillance center till considerably later. The scale of the loss owing theft and the like is therefore likely to increase beyond that had the intrusion been immediately discerned. This is a serious problem from the standpoint of system reliability
One object of the present invention is therefore to provide a security system using sensors that overcomes the aforesaid problems of the prior art.
Another object of the present invention is to provide a security system using sensors that has enhanced reliability.
Another object of the present invention is to provide a security system using sensors that achieves improved reliability by enabling all surveillance terminals of the system to share accurate time data.
The present invention achieves this object by providing a security system using sensors comprising: multiple surveillance terminals each equipped with a clock and adapted to send security and other data over a power supply line and a surveillance center for monitoring prescribed security zones utilizing security data received from the surveillance terminals, wherein at least one surveillance terminal among the multiple surveillance terminals uses the power supply line to send a time calibration signal to required ones of the other surveillance terminals at appropriate times, the surveillance terminals receiving the time calibration signal use the signal to correct their clocks to eliminate time error among the clocks of the multiple surveillance terminals, each surveillance terminal uses the power supply line to send to a prescribed surveillance terminal time-stamped data created by adding to the security and other data it sends send-time data indicating the time at which the data was sent, and the prescribed surveillance terminal discriminates operating states of the surveillance terminals by comparing receive time and send time of time-stamped data.
The operating state of a surveillance terminal can be assessed based on whether or not the time difference between the send time of the time-stamped data sent by the surveillance terminal and the time when the time-stamped data was actually received is greater than a prescribed value. The result of the assessment can be output by displaying it on an appropriate visual display means or announcing it as a voice message. Otherwise, a configuration can be adopted that sends the result of the assessment from the surveillance terminal to the surveillance center.
The clocks of the surveillance terminals are calibrated at appropriate times based on the time calibration signal. Since the surveillance terminals can therefore share the same time data, the time comparison can be made with very high, split-second order accuracy.
The accuracy of the time calibration can be enhanced by utilizing a standard radio signal carrying time data that traces back to a national standard. This can be achieved by installing in one of the surveillance terminals a receiver for receiving the standard radio signal, deriving a time calibration signal including standard time data from the standard radio signal, and sending the time calibration signal to the other surveillance terminals via the power supply line.
In another configuration of the security system, the individual surveillance terminals are equipped with power-line carrier (PLC) transceivers they use to exchange various data with one another over the power supply line.